adds dates validation to polls

app/views/admin/poll/polls/show.html.erb

@@ -1,8 +1,6 @@
-<%= render "shared/back_link" %>
-<div class="clear"></div>
-
 <h2 class="inline-block">
   <%= @poll.name %>
+  <small><%= l @poll.starts_at.to_date %> - <%= l @poll.ends_at.to_date %></small>
 </h2>
 
 <%= link_to t("admin.actions.edit"),